Primary Mid-ocean Ridge Basalts Research Articles

Heavy iron isotopes reveal mantle oxidation in addition to pyroxenite sources for intraplate basalts

The heavier iron isotopic composition of some oceanic island basalts (OIB) relative to primary mid-ocean ridge basalts (MORB) has commonly been interpreted to reflect the heavy bulk isotopic compositions of their sources (e.g., of a pyroxenite-dominated mantle region). Alternatively, comparable Fe isotopic signatures observed in continental intraplate basalts (CIB) may record enhanced isotope fractionation during partial melting of an oxidized source, offering valuable insights into mantle oxidation induced by subducted carbonate. To distinguish between these two independent contributions, here we present high-precision Fe isotopic data (δ57Fe) and Fe3+/∑Fe ratios for a suite of Cenozoic intraplate basalts from Inner Mongolia, East Asia. These basalts are distributed along a northwest-southeast (NW–SE) profile and associated with earlier subduction of the Paleo-Asian oceanic slab. Variations in Fe/Mn ratios and zinc isotopic compositions (δ66Zn) of these basalts reveal that altered oceanic crust has always contributed to their mantle sources but the amounts of recycled carbonate gradually vary. Together with existing data, we demonstrate that all of these basalts generally exhibit heavier δ57Fe (0.07 ‰ to 0.31 ‰), higher Fe3+/∑Femelt (0.08 to 0.39) and Fe/Mn (> 65) relative to primary MORB (δ57Fe ∼0.10 ‰; Fe3+/∑Femelt ∼0.10; Fe/Mn ∼60), pointing to a common origin of heavy Fe isotopes in intraplate basalts that may reflect those of pyroxenite sources (e.g., recycled crust). However, these basalts containing larger amounts of recycled carbonate in their sources (i.e., higher δ66Zn) have heavier δ57Fe at similar Fe/Mn ratios and proportions of pyroxenite melt. Such Fe isotopic signatures exceed the exclusive role of pyroxenite and are best attributed to partial melting at oxidizing conditions (i.e., high Fe3+/∑Fesource) with an inferred enlarged fractionation factor. This oxidized mantle source formed through the reduction of recycled carbonate to graphite/diamond at depths greater than 150 km. A global dataset of Fe isotopes in intraplate basalts is compiled in order to examine this interpretation further. Compared with OIB typically from a pyroxenite/eclogite-dominant source, CIB with similarly heavy δ57Fe and formed by similar melting degrees are commonly more silica-undersaturated, indicating a widespread carbonated mantle source. Thus, these observations highlight that heavy Fe isotopes can record carbonate-induced mantle oxidation in addition to source lithological heterogeneity for intraplate basalts.

Trace element geochemistry of mantle xenoliths from Zarnitsa kimberlite pipe, Daldyn field, Yakutia: Complex history of melts interactions with lithospheric mantle

Mantle xenoliths from Zarnitsa pipe studied in gray eruptive breccias (early) brown autholitic breccia (BAB) and black macrocrystic kimberlites (last dike; BMK), include garnet and spinel dunites-harzburgites, pyroxenites, eclogites, glimmerites and megacrysts. PT reconstructions using xenoliths reveal sharply layered structure (8 levels), estimated with the single grain mineral estimates mark hot (Cpx, Ilm) and cold (OPx, Gar) inflected geotherm. The interaction with plume melts is found at the lithosphere asthenosphere boundary (LAB), pyroxenite layer (3-4 GPa), Gar-Sp transition and Moho. Eclogites reveal Fe# growth from LAB to middle pyroxenites layer. Clinopyroxenes and ilmenite estimates marks melt refertilisations in interlayers between coupled subduction slabs. Source of capture from first to third stage deepening to LAB and Cr- rich garnets (to 19.5 Cr2O3) are below the LAB.The grey erupted breccia (GEB) includes mainly depleted and deformed peridotites. The later BAB includes pyroxenites, eclogites, and refertilised, deformed, and veined peridotites in BMK. Geochemistry of minerals changes from primary mid ocean ridge basalts (MORB) and back arc peridotites with low REE and large ion lithophile elements (LILE) and deeps in high field strength elements (HFSE) to metasomatized by alkaline (high Na and LILE) and adakitic melts (high Al, Na, Sr, and elevated HFSE) varieties and refertilised lherzolites due to plume at last stage. Mantle column metasomatized with scattered phlogopites in early grey eruptive breccia to amphibole-phlogopite ilmenite veins at last stages. Amphiboles trace mantle from lithosphere – asthenosphere boundary to Moho.Growth of the diamond grade from early eruptive breccia to later kimberlite phases refer to decreasing of the crust material and deepening of the xenoliths capture level. Metasomatism dissolve the diamonds but growth of megacrystic diamond crystals increase diamond grade.

Oceanic Zircon Records Extreme Fractional Crystallization of MORB to Rhyolite on the Alarcon Rise Mid-Ocean Ridge

Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.

Chromium isotope fractionation during magmatic processes: Evidence from mid-ocean ridge basalts

Chromium (Cr) isotopes represent a powerful tool for tracing the redox conditions during planetary magmatic evolution. However, so far, the systematic investigation of Cr isotope variation has been only performed on ocean island basalts (OIBs) in terrestrial magmatic rocks. Therefore, the Cr stable isotope compositions (expressed as δ53Cr relative to NIST SRM 979) of other magmatic rocks, formed under different oxygen fugacity, have remained unconstrained. In this paper, we present the first Cr stable isotopic data of mid-ocean ridge basalts (MORBs) from the Eastern Pacific Ocean ridge, the Indian ridge, and the Atlantic Ocean ridge. The oxygen fugacity of such basalts is different from that of the above-mentioned OIBs. The Rhyolite-MELTS model shows that the chemical composition variations in the studied basalts are induced by varying extents of fractional crystallization of olivine, clinopyroxene, plagioclase, and spinel. The δ53Cr values of the MORBs range from −0.27 ± 0.03‰ to −0.07 ± 0.02‰ (n = 28), and two distinct groups of basalts are identified based on the correlation between δ53Cr and MgO. On the one hand, the δ53Cr of group I basalts (−0.27 ± 0.03‰ to −0.14 ± 0.03‰; n = 24) are systematically lower than the established average value of the Bulk Silicate Earth (BSE) (δ53Cr = −0.12 ± 0.04‰; 2SD), which are positively correlated with their MgO and Cr concentrations, indicating that Cr isotopes are fractionated during magmatic differentiation. Moreover, Rayleigh fractionation modelling suggests that the crystallization of olivine, clinopyroxene, and spinel gives rise to the Cr depletion and thereby decreases δ53Cr values. On the other hand, group II basalts (−0.10 ± 0.03‰ to −0.07 ± 0.02‰; n = 4) exhibit higher δ53Cr values than group I with identical MgO and Cr concentrations. This is possibly associated with the crystallization of clinopyroxene under low pressure.The average Cr isotope composition (δ53Cr = −0.16 ± 0.02‰, n = 3) of the primitive basalts (MgO > 9%) represents that of the primary MORB melt, which is lighter than the average value of BSE. Using the non-modal melting equations, the δ53Cr of the MORB mantle source is estimated to be −0.12 ± 0.02‰ (2σ), which is consistent with that of BSE. Compared with the Cr isotopic data of OIBs from Fangataufa island, we find that the equilibrium fractionation factors (Δ53Crcrystal-melt = +0.04‰ to +0.13‰) of MORBs during fractional crystallization are larger than that of Fangataufa island lavas (Δ53Crcrystal-melt = 0.010 ± 0.005‰ for low-K suite, and 0.020 ± 0.010‰ for high-K suite; Bonnand et al., 2020a), indicating that the basalts from Fangataufa island have higher oxygen fugacity than those of MORBs analyzed in this study, which is strongly supported by their higher V/Sc ratios.

Sulfide in dunite channels reflects long-distance reactive migration of mid-ocean-ridge melts from mantle source to crust: A Re-Os isotopic perspective

The Earth's major heat and mass fluxes are output from the mantle to surface via mid-ocean-ridge-basalt (MORB) magmatism. The variations of MORBs are determined both by the heterogeneity of mantle source and the melt migration processes from mantle source to seafloor. However, before the MORB magmas enter crust, the role of the major melt migration passageways within the mantle (dunite channel systems) in regulating the compositions of MORBs has been rarely evaluated, especially from the perspective of Re-Os isotopes. Here, we report new Re-Os isotopic compositions of base-metal sulfides (BMS), chromites and dunites from dunite lenses with low spinel Cr# [Cr3+/(Cr3++Al3+) ≤0.66] (products of interaction between MORB-like melts and upper-mantle harzburgites) from the Zedang ophiolite (South Tibet). Rhenium-Os isotopic compositions of low-Cr# dunites from the Samail and Wadi Tayin massifs (typical MOR segments) of the Oman ophiolite are also shown for comparison to reveal the melt plumbing processes at mantle depths beneath mid-ocean ridges. Mineralogical evidence suggests that the Zedang sulfides and desulfurized alloy portions were originally precipitated as monosulfide solid solutions. The highly variable 187Os/188Os initial ratios (0.1191-0.1702) and low 187Re/188Os (<0.22) of the sulfides suggest that the chromite acted as a sink for Os-bearing sulfides, aggregating discrete Os components with heterogeneous isotopic signatures from asthenospheric or lithospheric mantle into dunite channels. The Zedang chromites and dunites show 187Os/188Os ratios similar to the primitive upper mantle (PUM), except for two dunites with sub-PUM ratios, reflecting the contribution of Os balanced by smaller volumes of Os-rich, unradiogenic sulfides (likely nucleating on Os nanoparticles) and larger volumes of Os-poor radiogenic BMS. Such isotopic heterogeneity, despite with less variation, has been observed in dunite channels from the Samail and Wadi Tayin massifs of the Oman ophiolite and present-day mid-ocean ridges. Formation of dunite channels in the upper mantle thus can aggregate Os-bearing sulfides with chromite, leaving high Re/Os components into the residual MOR melts. Once such channel systems were built up at the Moho transition zone, the newly incoming MOR magmas would preferentially dissolve the volumetrically abundant radiogenic BMS and retain Os-rich alloys/sulfides in the channels, further amplifying the Os-isotope mismatch observed between global oceanic crust (more radiogenic) and lithospheric mantle (less radiogenic).This study reveals that the long-distance reactive migration of melts from mantle source to at least Moho depths can result in strong Re-Os fractionation and isotopic mismatch between primary MORBs and mantle residues. It also shows that, unlike other lithophile tracers (e.g., Nd-Hf isotopes) whose signatures are almost completely transferred from mantle source to crust, the sub-crustal melt plumbing processes beneath mid-ocean ridges can be finely constrained by the multistage evolution of chalcophile and siderophile elements (e.g., Re-Os) and their isotopes (e.g., 187Re-187Os) with sulfides and chromites. Our study supports that the MORB compositions are actually regulated by their reactive migration processes occurred between melt generation at depth and eruption on seafloor.

Iron isotope fractionation during mid-ocean ridge basalt (MORB) evolution: Evidence from lavas on the East Pacific Rise at 10°30′N and its implications

Whether the Earth’s mantle has a chondritic δ56Fe (deviation in 56Fe/54Fe from the IRMM-014 standard in parts per thousand) value or not remains under debate. The current view is that the observed average δ56Fe of mid-ocean ridge basalts (MORB) cannot be explained by partial melting of mantle source with chondritic value alone. Here, we report Fe isotope compositions on 29 MORB glasses sampled along a flowline traverse across the East Pacific Rise (EPR) axis at 10°30′N. These glasses show large MgO variation (1.8–7.4 wt.%) that forms a compositional continuum resulting from varying extent of fractional crystallization, which is accompanied by systematic Fe isotopic variation. Fractional crystallization modeling suggests that early crystallization of olivine, pyroxene and plagioclase gives rise to an iron enrichment trend and an increase in δ56Fe. Once Fe-Ti oxides appear on the liquidus and begin to crystallize, the FeOt and TiO2 contents of the residual melt decrease rapidly, which lead to a slight decrease in δ56Fe. These observations indicate that significant Fe isotope fractionation can indeed take place during MORB melt evolution. Hence, δ56Fe values of variably evolved MORB melts do not represent those of primary MORB melts and thus cannot be used to infer mantle source Fe isotope compositions. Importantly, δ56Fe values of primary MORB melts after correction for the effect of fractional crystallization can be well reproduced by mantle melting. Therefore, our study supports the idea that the Fe isotope composition of the accessible Earth is close to be chondritic. We note that conclusion would assume that the core, which takes up ∼90% of the Earth’s Fe, must have a chondritic Fe isotope composition.

Transition from ultra-enriched to ultra-depleted primary MORB melts in a single volcanic suite (Macquarie Island, SW Pacific): Implications for mantle source, melting process and plumbing system

Compositional diversity of basalts forming the oceanic floor is attributed to a variety of factors such as mantle heterogeneities, melting conditions, mixing of individual melt batches, as well as fractionation and assimilation processes during magma ascent and emplacement. In this study the compositional range and origin of mid-ocean ridge basalts (MORB) is approached by petrological, mineralogical and geochemical studies of the Miocene Macquarie Island ophiolite, an uplifted part of the Macquarie Ridge at the boundary between the Australian and Pacific plates. In this study, earlier results on the enriched to ultra-enriched (La/Sm 1.4–7.9), isotopically homogeneous basaltic glasses are complemented by the compositions of olivine-phyric rocks, principal phenocrystic minerals and Cr-spinel hosted melt inclusions. Studied olivine, clinopyroxene and Cr-spinel phenocrysts are among the most primitive known for MORB (85–91mol% forsterite in olivine, 81–91 Mg# in clinopyroxene, and 66–77 Mg# and 34–60 Cr# in spinel) and represent primary and near-primary compositions of their parental melts. Geochemical characteristics of the liquids parental to clinopyroxene (La/Sm 0.8–6.3) and Cr-spinel (La/Sm 0.4–5) partly overlap with those of the basaltic glasses, but also strongly advocate the role of depleted to ultra-depleted primary melts in the origin of the Macquarie Island porphyritic rocks. The trace element composition of olivine phenocrysts and the systematics of rare-earth elements in glasses, melt inclusions, and clinopyroxene provide evidence for a peridotitic composition of the source mantle. Our data supports the mechanism of fractional “dynamic” melting of a single mantle peridotite producing individual partial melt batches with continuously changing compositions from ultra-enriched towards ultra-depleted. The incipient enriched melt batches, represented by basaltic glasses in this study, may erupt without significant modification, whereas consecutively derived less enriched and depleted melt fractions are affected by mixing and crystal fractionation. Continuous melt generation, extraction, replenishment of the plumbing system, mixing and eruption of “integrated” melts lead to obliteration of the initially enriched geochemical characteristics and ultimately result in dominantly “normal”, depleted MORB compositions.

The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle

Micro-analytical determination of Fe3+/∑Fe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure (μ-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe3+/∑Fe ratios of 0.16±0.01 (n=103), which trace back to primary MORB melts equilibrated at the conditions of the quartz–fayalite–magnetite (QFM) buffer. Our results necessitate an upward revision of the Fe3+/∑Fe ratios of MORBs, mantle oxygen fugacity, and the ferric iron content of the mantle relative to previous wet chemical determinations. We show that only 0.01 (absolute, or <10%) of the difference between Fe3+/∑Fe ratios determined by micro-colorimety and XANES can be attributed to the Mössbauer-based XANES calibration. The difference must instead derive from a bias between micro-colorimetry performed on experimental vs. natural basalts. Co-variations of Fe3+/∑Fe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe3+ behaving incompatibly in shallow MORB magma chambers. MORB Fe3+/∑Fe ratios do not, however, vary with indices of the extent of mantle melting (e.g., Na2O(8)) or water concentration. We offer two hypotheses to explain these observations: The bulk partition coefficient of Fe3+ may be higher during peridotite melting than previously thought, and may vary with temperature, or redox exchange between sulfide and sulfate species could buffer mantle melting at ~QFM. Both explanations, in combination with the measured MORB Fe3+/∑Fe ratios, point to a fertile MORB source with greater than 0.3wt.% Fe2O3.

Graphite-melt equilibria during mantle melting: constraints on CO 2 in MORB magmas and the carbon content of the mantle

Melts produced under fluid-absent, graphite-saturated conditions have tightly constrained oxidation states and CO 2 contents. A model is presented for the generation of mid-ocean ridge basalt (MORB) magma by fluid-absent partial melting of graphite-saturated upper mantle. The model results in melt ferric/ferrous ratios and oxygen fugacities which are consistent with observed values in glassy MORBs and oceanic peridotites. Increasing the degree of partial melting causes decreases in the ferric/ferrous ratio of the melt, the oxygen fugacity, and the concentration of CO 2 in the melt. Sensitivity analysis indicates that the initial ferric-iron concentration in the source region has the greatest effect on the results. For the range of probable initial values, the oxygen fugacity of the graphite-saturated primary MORB magma at 1 GPa is 2.5 log 10 units below the Ni–NiO buffer, and the CO 2 content of the melt ranges from 900 to 1800 ppm (mass). The amount of source region carbon consumed during partial melting is between 40 and 80 ppm. The dissolved CO 2 in MORBs represents a flux of 3.5 to 7×10 10 kg/year which is significantly greater than estimates based on the measured CO 2 content of glassy MORB samples.

Recent volcanism in the Siqueiros transform fault: picritic basalts and implications for MORB magma genesis

Small constructional volcanic landforms and very fresh-looking lava flows are present along one of the inferred active strike-slip faults that connect two small spreading centers (A and B) in the western portion of the Siqueiros transform domain. The most primitive lavas (picritic and olivine-phyric basalts), exclusively recovered from the young-looking flows within the A-B strike-slip fault, contain millimeter-sized olivine phenocrysts (up to 20 modal%) that have a limited compositional range (Fo 91.5-Fo 89.5) and complexly zoned CrAl spinels. High-MgO (9.5–10.6 wt%) glasses sampled from the young lava flows contain 1–7% olivine phenocrysts (Fo 90.5-Fo 89) that could have formed by equilibrium crystallization from basaltic melts with Mg# values between 71 and 74. These high MgO (and high Al 2O 3) glasses may be near-primary melts from incompatible-element depleted oceanic mantle and little modified by crustal mixing and/or fractionation processes. Phase chemistry and major element systematics indicate that the picritic basalts are not primary liquids and formed by the accumulation of olivine and minor spinel from high-MgO melts (10% < MgO < 14%). Compared to typical N-MORB from the East Pacific Rise, the Siqueiros lavas are more primitive and depleted in incompatible elements. Phase equilibria calculations and comparisons with experimental data and trace element modeling support this hypothesis. They indicate such primary mid-ocean ridge basalt magmas formed by 10–18% accumulative decompression melting in the spinel peridotite field (but small amounts of melting in the garnet peridotite field are not precluded). The compositional variations of the primitive magmas may result from the accumulation of different small batch melt fractions from a polybaric melting column.

Refertilization in a Convecting Mantle: Effects on Primary MORB Compositions

The widely held view is that mid-ocean ridge basalts (MORBs) are produced by polybaric fractional melting of the sub-oceanic mantle in which the composite primary MORBs are derived from a relatively fertile mantle by 10-25% total melting (McKenzie, 1985; Johnson et al., 1990). The differences in primary MORB compositions are inferred to result principally from differences in the pressure and extent of melting (Klein and Langrnuir, 1987; Kinzler and Grove, 1992). The compositional variations between abyssal peridorites, in this scenario, reflect different degrees of fractional melting in which the least depleted peridotites represent the residues of ~10% melting and the most depleted peridotites represent the residues of ~25% melting (Dick and Fisher, 1984; Michael and Bonatti, 1985). It has been suggested, however, that the abundances of moderately incompatible elements (e.g., Na) in abyssal peridotites are not compatible with these rocks having been formed as residues of variable degrees of partial melting (Elthon, 1992). It was suggested instead that abyssal peridotites are a mixture of two distinct components. One component is a depleted harzburgite that has either no clinopyroxene (CPX) or that has < 2% CPX with very low abundances of Na20 (0.01 wt. %), Ti (300 ppm), Zr (0.1 ppm), and Ce (0.02 • chondrites). The other component is a basaltic liquid, which is potentially highly variable in composition. If the chemical variations in abyssal peridotites reflect some type of melt addition (or refertilization) process into depleted harzburgite mantle, there are profound implications for how the source regions of MORBs have evolved. Most importantly, if the mantle melts to produce a harzburgite (< 2% CPX) on a routine basis, the differences in primary composite MORBs will reflect most strongly the differences in the local mantle composition as the mantle enters the midocean ridge melting regime. In light of the variable compositions of mantle xenoliths and peridotite massifs as well as the strong evidence for melt addition processes in these samples, it might also be expected that the sub-oceanic mantle is both variable in composition and is influenced by meltaddition (refertilization) processes. It is proposed here that the MORB source region has been through the mid-ocean ridge melting regime at least 1 to 5 times during normal mantle evolution and consists of a highly depleted harzburgite mantle (similar to that found in peridotite massifs) mixed with basaltic/komatiitic material that has refertilized the mantle. In this scenario, the ascending mantle beneath midocean ridges varies significantly in composition, depending on the amount and type of basaltic/ komatiitic mantle added. Within the melting regime, the mantle melts to whatever extent is necessary to deplete the mantle to harzburgite (+ a few percent clinopyroxene) where thermal considerations stop further melting. It is proposed, therefore, that differences in primary MORB compositions are principally due to differences in the amount and composition of basaltic/komatiitic material added into the source region.

An empirical method for calculating melt compositions produced beneath mid‐ocean ridges: Application for axis and off‐axis (seamounts) melting

We present a new method for calculating the major element compositions of primary melts parental to mid‐ocean ridge basalt (MORB). This model is based on the experimental data of Jaques and Green (1980), Falloon et al. (1988), and Falloon and Green (1987, 1988) which are ideal for this purpose. Our method is empirical and employs solid‐liquid partition coefficients (Di) from the experiments. We empirically determine Di = ƒ(P,F) and use this to calculate melt compositions produced by decompression‐induced melting along an adiabat (column melting). Results indicate that most MORBs can be generated by 10–20% partial melting at initial pressures (P0) of 12–21 kbar. Our primary MORB melts have MgO = 10–12 wt %. We fractionate these at low pressure to an MgO content of 8.0 wt % in order to interpret natural MORB liquids. This model allows us to calculate Po, Pƒ, To, Tƒ, and F for natural MORB melts. We apply the model to interpret MORB compositions and mantle upwelling patterns beneath a fast ridge (East Pacific Rise (EPR)8°N to 14°N), a slow ridge (mid‐Atlantic Ridge (MAR) at 26°S), and seamounts near the EPR (Lament seamount chain). We find mantle temperature differences of up to 50°–60°C over distances of 30–50 km both across axis and along axis at the EPR. We propose that these are due to upward mantle flow in a weakly conductive (versus adiabatic) temperature gradient. We suggest that the EPR is fed by a wide (−100 km) zone of upwelling due to plate separation but has a central core of faster buoyant flow. An along‐axis thermal dome between the Siqueiros transform and the 11°45′ Overlapping Spreading center (OSC) may represent such an upwelling; however, in general there is a poor correlation between mantle temperature, topography, and the segmentation pattern at the EPR. For the Lament seamounts we find regular across‐axis changes in Po and F suggesting that the melt zone pinches out off axis. This observation supports the idea that the EPR is fed by a broad upwelling which diminishes in vigor off axis. In contrast with the EPR axis, mantle temperature correlates well with topography at the MAR, and there is less melting under offsets. The data are consistent with weaker upwelling under offsets and an adiabatic temperature gradient in the sub axial mantle away from offsets. The MAR at 26°S exhibits the so‐called local trend of Klein and Langmuir (1989). Our model indicates that the local trend cannot be due solely to intracolumn melting processes. The local trend seems to be genetically associated with slow‐spreading ridges, and we suggest it is due to melting of multiple individual domains that differ in initial and final melting pressure within segments fed by buoyant focused mantle flow.

Pressure of origin of primary mid-ocean ridge basalts

Summary High-pressure phase equilibrium studies of mid-ocean ridge basalts (MORBs) have shown that some samples are in equilibrium with olivine + orthopyroxene (OPX) + clinopyroxene ± plagioclase at about 10 kbar, whereas others are not in equilibrium with OPX at this pressure. Samples that are OPX-saturated at about 10 kbar are characterized by higher SiO 2 (&gt;49.7 wt%) than most (about &gt;75%) MORB glasses with more than 9.5 wt% MgO, suggesting that only about 25% of these glasses could be derived from primary magmas generated at about 10 kbars. Orthopyroxenes in abyssal peridotites are characterized by Al 2 O 3 contents that range from 2.0 to 6.0 wt% (probably about 2.5–6.5 wt% before subsolidus equilibration), but orthopyroxenes in peridotite partial-melting experiments and liquidus orthopyroxenes for MORBs at about 10 kbar have only 3.5–4.3 wt% Al 2 O 3 . Liquidus orthopyroxenes for high-MgO basalts at 25 kbar and 20 kbar have about 6.6 wt% and 5.5 wt% Al 2 O 3 respectively. It appears that the least depleted (Al 2 O 3 -rich end) of abyssal peridotite orthopyroxenes indicate the dominance of melting and primary MORB genesis at pressures substantially greater than 10 kbar, and probably at 20–25 kbar.

Mariana Trough lavas from 18°N: Implications for the origin of back arc basin basalts

Geochemical data on glass separates from four dredges and two Deep Sea Drilling Project sites near 18°N in the Mariana Trough define 17 distinct chemical groups that probably represent individual magma batches. Much of the observed variation within the sample suite can be explained by low‐pressure fractionation, although effects of magma mixing and variations in melt‐generating processes are also apparent. Relative to mid‐ocean ridge basalts (MORB), Mariana Trough glasses have higher H2O and Al2O3 and lower FeO* at moderate degrees of differentiation. The high Al2O3 is a consequence of the displacement of the olivine‐plagioclase cotectic to lower MgO in the hydrous Mariana Trough magmas. Low FeO can be attributed to low T and P of melting of diapirs rising along cooler adiabats than those beneath normal mid‐ocean ridges. Extents of melting giving rise to Mariana Trough primary magmas were about 15%, within the range of melting extents for primary MORB. Relative to MORB, Mariana Trough lavas are enriched in Ba&gt;Rb&gt;K&gt;light rare earth elements&gt;Sr and depleted in Y. The degree of enrichment inversely correlates with solid/liquid distribution coefficients, suggesting control by a low‐degree melt component in the Mariana Trough source. Relative to submarine lavas of the Mariana arc, Mariana Trough volatiles are depleted in halogens. Melting of the upper part of the subducted crust could occur 125–175 km beneath the Mariana region by addition of volatiles derived from the breakdown of hydrous phases in the lower crust and upper mantle at depths of 150–210 km. Addition of such a melt component to the source region for Mariana Trough magmas will not quantitatively account for the incompatible element enrichments unless altered crust or crustal sediments are included in the slab being melted or open system hybridization reactions occur in the overlying mantle wedge.

Comments on ?Composition and depth of origin of primary mid-ocean ridge basalts? by D.C. Presnall and J.D. Hoover

Presnall and Hoover (1984), in their evaluation of the nature of primary mid-ocean ridge basalts (MORBs), concluded that earlier data suggesting many of the most primitive MORBs are derived from high-MgO (picritic) magmas are not valid. In reaching this conclusion, Presnall and Hoover (1984) ignore several important considerations regarding 1. analytical uncertainties, 2. TiO2abundances in MORBs, 3. least-squares calculations, 4. complex models of magmatic evolution and 5. enstatite saturation. I briefly discuss these points below.

Composition and depth of origin of primary mid-ocean ridge basalts ?reply to D. Elthon

Composition and depth of origin of primary mid-ocean ridge basalts ?reply to D. Elthon

The very depleted nature of certain primary mid-ocean ridge basalts

Many mid-ocean ridge basalts (MORBs) might be ultimately derived from primary magmas that are very depleted in Na 2O and TiO 2. These very depleted primary magmas have 0.60 to 1.50 wt.% Na 2O and 0.10 to 0.50 wt.% TiO 2 compared to MORBs, which typically have > 1.90% Na 2O and >0.60% TiO 2. Evidence for these depleted primary magma compositions is obtained from megacrysts in MORBs, from glass inclusions within these megacrysts, and from the highly calcic plagioclases (An 91–96) and depleted clinopyroxenes (Na 2O mostly between 0.10 and 0.35) in certain abyssal peridotites. Cumulate ultramanfi and gabbroic rocks from the North Arm Mountain Massif of the Bay of Islands ophiolite complex show a progressive increase in the Na 2O and TiO 2 abundances in clinopyroxene crystals with stratigraphic height in the ophiolite. The use of mineral-liquid distribution coefficients and cumulate mineral compositions indicate that the liquids from which these minerals crystallized had 0.10 to 0.20 wt.% TiO 2 and 0.60 to 0.80 wt.% Na 2O for the lowermost cumulate ultramafic rocks, with TiO 2 and Na 2O abundances of liquids increasing progressively to normal MORB abundances during crystallization of higher-level gabbroic cumulates. These data clearly demonstrate that primary basalts that are very depleted in Na 2O and TiO 2 can differentiate to form residual magmas that are indistinguishable from MORBs.

Composition and depth of origin of primary mid-ocean ridge basalts

Some workers have held that mid-ocean ridge basalts are fractionated from high pressure (15–30 kbar) picritic primary magmas whereas others have favored primary magmas generated at about 10 kbar with compositions close to those of mid-ocean ridge basalts. Of critical significance are presumed differences in composition between experimentally determined primary magmas and the least fractionated mid-ocean ridge basalts. To evaluate the significance of these differences, all based on electron microprobe analyses, we consider three sources of uncertainty: (1) analytical uncertainties for a single microprobe laboratory, (2) systematic interlaboratory analytical differences, and (3) real variations in the possible compositions of primary magmas that can be produced from a peridotite source at a given pressure. The first source of error is surprisingly large and can account for a substantial part of the total variation of normative quartz (hypersthene calculated as equivalent olivine and quartz) in FAMOUS basalts. The second is not as serious but remains undetermined for many laboratories. The third is potentially the largest but is not yet fully documented. The least fractionated FA-MOUS basalts have high mg numbers (70–73) compatible with derivation from the mantle by direct partial melting with little or no subsequent fractional crystallization. Because of the wide range of normative quartz content in these basalts, it appears necessary to consider them as representatives of multiple parental magmas. When all the sources of uncertainty are taken into account, we conclude that the experimental data by various investigators are all fairly consistent and favor derivation of the least fractionated mid-ocean ridge basalts by at most only a small amount of fractional crystallization from primary magmas having a wide range of normative quartz content and generated over a range of pressures from about 7–11 kbar.

Comment on “MORB—A Mohole misbegotten?” by M.J. O'Hara

One can only applaud O'Hara's call for further research into magmatic processes that take place at and near oceanic spreading centers (EOS, June 15, 1982). In particular, the desirability of having deep drill holes close to active spreading centers is indisputable and needs no elaboration. One of the major reasons for encouraging further research on midocean ridge basalt (MORB) petrogenesis is the need to resolve a controversy now being debated in the literature over the composition and depth of origin of primary MORB magmas [Rhodes, 1982]. O'Hara [1968a] and, recently, several others [Green et al., 1979; Stolper, 1980; Elthon and Scarfe, 1980] have argued that primary MORB magmas are picritic in composition and generated at various pressures from 15 to 30 kbar, depending on the author. Others contend that primary MORB magmas are close in composition to the least‐fractionated MORB glasses (which are not picritic but have mg numbers as high as 73) and are generated at more moderate pressures of about 8–10 kbar [Green and Ringwood, 1967; Kushiro, 1973; Presnall et al., 1979]. O'Hara did not mention any of the recent papers that disagreed with his concept of picritic primary magmas at spreading centers. In order to restore some balance, the following brief discussion is offered.

Geochemical characteristics of mid-ocean ridge basalts

Major and trace element (Rb, Sr, Y, Zr, Nb, Ba, Sc, Ni, Co, Cr, V) data are presented on ten mid-ocean ridge basalts (MORB), together with a basalt sample from the offshore slope of the Yap Trench and a basalt from the Tertiary ophiolite complex of eastern Taiwan. Rare earth element (REE) and Sr isotope data are presented on eight of the samples, together with REE data on U.S.G.S. standard rock, BHVO-1. A strong correlation exists between percent TiO 2 (proportional to amount of melting) and Al 2 O 3 /TiO 2 , CaO/TiO 2 ratios of these close to primary MORB. These ratios increase up to a maximum (about 20 and 17 respectively) as TiO 2 decreases, indicating a progressive release of Al and Ca from the mantle source. The limiting values of the ratios are close to chondritic and are reached at about 0.8% TiO 2 . At this high degree of melting, the major Al- and Ca-bearing mineral phases are eliminated from the mantle residue. Model calculations on the major element data indicate that MORB with 0.7% and 1.5% TiO 2 could represent about 25 and 15% melting (respectively). This modelling is consistent with the abundances of the first transition series metals (Sc to Ni), which are also interpretable in terms of degree of partial melting and residual mineralogy. Based on the REE data, the samples can be divided into depleted types (typical of “normal” MORB), and enriched types (the so-called “plume” and “transitional” types). Despite the range of (La/Sm) N in these samples (0.38–1.97) the Ti/Zr ratios remain close to chondritic (about 110) indicating that these two elements have a similar degree of incompatibility at this level of melting. Chondritic normalized patterns for typical “normal” (N)- and “plume” (P)-type MORB are presented for the REE and K, Nb, U, Th, Ba, Rb and Cs and it is suggested that this represents an increasing order of element incompatibility in MORB. The behaviour of Y, Ti, Zr, P and Sr relative to the REE is also discussed and it is shown that for most primitive MORB, Y = Ho, Ti = Eu, Zr = Sm, P = Nd and Sr is between Ce and Nd. In addition the Zr/Nb ratio correlates with the La/Sm ratio. This element correlation can be used to predict the general shape of REE patterns. The Sr isotope data are discussed in terms of already published data from the three oceans. It is suggested that P-type MORB originate from depleted mantle sources which only recently (say ≤300 m.y.) were added to by an incompatible-element-rich phase. Various models proposed to explain both the depletion in N-type, enrichment in the P-type MORB sources and the proposed mantle heterogeneity are discussed. It is concluded that mantle depletion is due to a continuing process rather than an episodic event in the early Archaean or 1.6 b.y. ago.